The present invention relates generally to the processing of clay and, more particularly, to a method for concentrating a beneficiated aqueous kaolin clay slurry by evaporating water therefrom using indirect heat exchange.
Kaolin clay has many known applications in industry, including use as a filler in paper making, a coating for paper, and a pigment in paint. However, crude kaolin clay typically contains various impurities which cause discoloration. Additionally, crude kaolin clay by various well-known commercial processes which increase the brightness of the kaolin by removing discoloration impurities and decrease the abrasiveness by reducing the particle size of the kaolin particles.
In general, such processes for beneficiating crude kaolin clay require that the clay be processed as a low solids slurry. Therefore, it is necessary to add substantial amounts of water to the dry crude kaolin clay to form a clay suspension or slurry having a low solids content, typically in the range of 15% to 40% by weight. However, for commercial applications, the beneficiated clay slurry must have a much higher solids content. Typically beneficiated kaolin clays are shipped commercially for use in paper making, paper coating and paint making as a high solids slurry having a solids content in the range of 65% to 75% by weight. Therefore, most of the water added to the dry kaolin clay must be removed in order to concentrate the clay solids.
In a typical conventional process for dewatering a beneficiated clay slurry, the low-solids slurry is typically first passed to a vacuum or press type filter wherein a limited portion of the water is removed from the slurry. Typically, the filter cake from the filter would have a solids content of about 50% to 60% by weight. Thus, the slurry would still comprise about 40% to 50% water. Further dewatering on a vacuum or press type filter is impractical due to the fine particle size of the solids in the beneficiated clay slurry. Typically, to further dewater the beneficiated clay slurry to a commercially acceptable solids content, at least a portion of the partially dewatered slurry is passed through a spray dryer or other direct contact-type evaporator such as a gas-fired kiln, wherein the clay slurry is contacted with a drying medium having a temperature of 1000.degree. F. or more, such as hot air or hot flue gas typically generated from the combustion of natural gas. Although all of the clay slurry may be passed through the spray dryer for drying, it is customary to pass only a portion of the clay slurry through the spray dryer and then to re-mix the thoroughly dried clay slurry from the spray dryer with the remaining portion of partially dewatered slurry in a high shear mixer to produce a product clay slurry having a solids content of 65% to 75%.
A problem encountered in concentrating clay slurries in spray dryers or other direct contact-type evaporators is the formation of agglomerates of dried clay during direct contact evaporation. Therefore, it is often necessary to pass the product clay slurry through a pulverizer in order to breakup such agglomerates prior to shipping the slurry. Additionally, when kaolin clays are dried in direct contact-type evaporators such as spray dryers at these high temperatures, the brightness of the clay particles deteriorate slightly. Further, spray drying is a relatively inefficient process and considerable energy is consumed in the spray drying process in order to evaporate the water in the clay slurry.
One very effective method of concentrating kaolin clay slurries by evaporating water therefrom in such a manner as to avoid the formation of agglomerates and the deterioration of clay brightness attendant to spray drying is disclosed in commonly assigned U.S. Pat. No. 4,687,546 of Willis. As disclosed therein, an aqueous beneficiated clay slurry is concentrated by evaporating water therefrom by passing the aqueous clay slurry through one or more non-contact evaporative heat exchangers in indirect heat exchange relationship with a heating vapor wherein the heating vapor comprises water vapor previously evaporated from the aqueous clay slurry. In this manner, an energy efficient process is provided for concentrating a beneficiated aqueous clay slurry in that the present invention makes use of the heat normally wasted when the flue gas from the spray dryer together with the water vapor evaporated from the clay during the spray drying process is vented to the atmosphere. Further, by using indirect heat exchange between the aqueous clay slurry and the heating vapor as a means of evaporating water vapor from the clay slurry, the clay and the hot drying vapor do not contact, thereby avoiding, formation of agglomerates typically encountered in the direct contact evaporators.
In one embodiment disclosed in U.S. Pat. No. 4,687,546, a continuous stream of clay slurry to be concentrated is passed through a single non-contact type evaporative heat exchanger in indirect heat exchange relationship with recycled water vapor. That is, water vapor evaporated from the clay slurry in the heat exchanger is collected, compressed to increase its temperature, and recycled to the heat exchanger as the heating vapor to evaporate water from the incoming clay slurry.
In another embodiment disclosed in U.S. Pat. No. 4,687,547, a continuous stream of the clay slurry to be concentrated is passed through a plurality of non-contact evaporative heat exchangers in series flow from the upstream-most of the heat exchangers to the downstream-most of the heat exchangers in indirect heat exchange relationship with a heating vapor. The heating vapor in each of the evaporative heat exchangers comprises the water vapor evaporated from the aqueous clay slurry in the adjacent downstream evaporative heat exchanger, except in the downstream-most of the evaporative heat exchangers wherein the heating vapor is supplied from an independent source. The aqueous clay slurry exiting the downstream-most evaporative heat exchanger may be passed through a flash tank wherein additional water is removed from the aqueous clay slurry thereby further concentrating the solids in the aqueous clay slurry. Additionally, it is disclosed that the aqueous clay slurry to be concentrated may be preheated by passing the aqueous clay slurry in indirect heat exchange relationship with the water vapor evaporated from the aqueous clay slurry in the upstream-most evaporative heat exchanger prior to passing the aqueous clay slurry to the upstream-most evaporative heat exchanger.
However, in some clay processing operations a heating vapor, such as steam, may not be readily available for initiating the evaporation process in the vapor driven indirect evaporative drying process as disclosed in U.S. Pat. No. 4,687,546, whether it be a single-effect or multi-effect embodiment of the process. Rather, hot liquid, typically water having a temperature in the range of about 130.degree. F. to about 180.degree. F., may be the only heating medium readily available. Therefore, it would desirable to be able to utilize such moderate temperature hot liquid as the driving fluid, i.e., heating medium, to concentrate solids in an aqueous clay slurry by passing the aqueous clay slurry in indirect, non-contact heat exchange relationship with the hot liquid to evaporate water from the aqueous clay slurry.
Accordingly, it is the general object of the present invention to provide a method for concentrating a beneficiated aqueous clay slurry in an energy efficient manner by evaporating water from the clay slurry using hot liquid as the heating medium.
It is a further object of the present invention to provide a method for concentrating the beneficiated aqueous clay kaolin slurry by evaporation without the formation of agglomerates or the deterioration of clay brightness during the drying process.